1. Field of the Invention
The present invention relates to an integrator, and more particularly to an integrator for use in a switched capacitor-filter or the like.
2. Description of the Prior Art
It is well-known in the art that in a switched capacitor-filter, such as integrator as shown in FIG. 1 is used as a fundamental unit of the filter (see, for example, IEEE Journal of Solid-State Circuits, Vol. SC-12, No. 6, December 1977, pp. 600-608.). In FIG. 1, the integrator comprises a switched capacitor, including a capacitor CA and a switching element S for periodically switching the capacitor CA to input and output sides a and b. The integrator also includes an operational amplifier OP having a first input connected to the output side b of the switching element S and having the second input connected to a reference potential source, in the illustrated example, a ground, and an integrating capacitor CB connected between the first input and the output of the operational amplifier OP. The operation of this integrator is as follows: In an ideal situation the operational amplifier OP has infinite DC gain and input impedance and zero output impedance, and if the operational amplifier OP is assumed to be ideal, then the potential at a point A is zero. When connecting the switching element S to the input side a when t=0, letting an input voltage V.sub.IN at that time be represented by V.sub.IN0, charges Q.sub.A =V.sub.IN0 .multidot.CA are stored in the integrating capacitor CB. Next, when the switching element S is switched to the output side b from the input side a at t=t.sub.1, the charges Q.sub.A =V.sub.IN0 .multidot.Ca are connected to the point A and thence are applied to the capacitor CA. Accordingly, the output voltage Vout from the operational amplifier OP long after the moment t=t.sub.1 becomes as follows: ##EQU1## where Vout.sub.0 is an output voltage of the operational amplifier OP at the moment of switching the switching element S to the output side b. The reason is that since the output of the operational amplifier OP is fed back to its input side, the potential at the point A is zero as described above and if the potential at the point A is zero, then charges Q.sub.A of the capacitor CA are all transferred to the integrating capacitor CB, making its potential equal to the output voltage Vout. Repeating the above operation of connecting the switching element S to the input side a and then to the output side b, th output voltage Vout becomes as follows: ##EQU2## where n is the number of times the switching element S is switched from a to b. In the above expression, V.sub.IN0 is constant and if equation (2) is a function of time, then the second term of the expression becomes an integral expression. While this integrator continuously operates, switching the switching element S between the input and output sides a and b, there is no problem, but when the integrator is out of operation, the following problem arises: That is, while the integrator is out of operation, what is called a power-down procedure is taken with a view toward avoiding power consumption. The power-down procedure is carried out by turning OFF entirely or partly the power source of the operational amplifier OP in order to stop its function. As a consequence, the function of holding the point A at the potential of the other input terminal, in this example, at a zero level is lost and the point A assumes a variable potential depending on a leakage current, induction current or the like. When the potential at the point A becomes variable, for example, a positive or negative high potential while the integrator is held out of operation for a long time, it is necessary at the start of an integral operation, the above-mentioned high potential be lowered to the zero level via the negative feedback circuit including the integrating capacitor. Because the potential must be lowered much time is required until the potential at the point A becomes stable.